Sharing a public page

Our Website isn’t public, we have got users with Login, but we want to share Posts by E-Mail, Twitter and Facebook for the public. In a older Version, the program creates a URL with a public-page-link. The person who receives the link can see only the post and links in the post and not the whole website. The URL Looks like URL of the Website/publicpage?post=90a3114638437e7l5.
How can we create and add such a link by sharing the article?